New flood-tolerant rice offers relief for world's poorest farmers

A gene that enables rice to survive complete submergence has been
identified by a team of researchers at the International Rice
Research Institute in the Philippines and at the University of
California's Davis and Riverside campuses. The discovery allows for
development of new rice varieties that can withstand flooding, thus
overcoming one of agriculture's oldest challenges and offering relief
to millions of poor rice farmers around the world.

While rice thrives in standing water, like all crops it will die if
completely submerged for more than a few days. The development and
cultivation of the new varieties is expected to increase food
security for 70 million of the world's poorest people, and may reduce
yield losses from weeds in areas like the United States where rice is
seeded in flooded fields. Results of this study will appear in the
Aug. 10 issue of the journal Nature.

"Globally, rice is the most important food for humans, and each year
millions of small farmers in the poorest areas of the world lose
their entire crops to flooding," said Pamela Ronald, a rice
geneticist and chair of UC Davis' Plant Genomics Program. "Our
research team anticipates that these newly developed rice varieties
will help ensure a more dependable food supply for poor farmers and
their families. And, in the long run, our findings may allow rice
producers in the United States to reduce the amount of herbicides
used to fight weeds."

Background

Rice is the primary food for more than 3 billion people around the
world. Approximately one-fourth of the global rice crop is grown in
rain-fed, lowland plots that are prone to seasonal flooding. These
seasonal flash floods are extremely unpredictable and may occur at
any growth stage of the rice crop.

While rice is the only cereal crop that can withstand submergence at
all, most rice varieties will die if fully submerged for too long.
When the plant is covered with water, its oxygen and carbon dioxide
supplies are reduced, which interferes with photosynthesis and
respiration. Because the submerged plants lack the air and sunlight
they need to function, growth is inhibited, and the plants will die
if they remain under water for more than four days.

During any given year, yield losses resulting from flooding in these
lowland areas may range from 10 percent to total destruction,
depending on the water depth, age of the plant, how long the plants
are submerged, water temperature, rate of nitrogen fertilizer use and
other environmental factors. Annual crop loss has been estimated at
more than $1 billion.

"For half a century, researchers have been trying to introduce
submergence tolerance into the commonly grown rice varieties through
conventional breeding," said rice geneticist and study co-author
David Mackill, who heads the Division of Plant Breeding, Genetics,
and Biotechnology at the International Rice Research Institute.
"Several traditional rice varieties have exhibited a greater
tolerance to submergence, but attempts to breed that tolerance into
commercially viable rice failed to generate successful varieties.

"We're especially pleased that we have been able to use the latest
advances in molecular biology to help improve the lives of the
world's poor," Mackill added. "We're confident that even more
important discoveries like this are in the pipeline."

Results of this study

Using genetic mapping techniques, the research team identified a
cluster of three genes that appeared closely linked to the biological
processes that either make rice plants vulnerable to flooding or
enable them to withstand the total submergence that occurs during
flooding.

The researchers then focused their attention on one of those three
genes, known as the Sub1A gene. They found that when this gene is
over-expressed, or hyper-activated, a rice variety that is normally
intolerant of submergence becomes tolerant.

Further studies indicated that the Sub1A gene is likely successful in
conferring submergence tolerance to rice because it affects the way
the plants respond to hormones, such as ethylene and giberellic acid,
that are key to the plant's ability to survive even when inundated
with water.

Going one step further, the researchers introduced the Sub1A gene
into a rice variety that is especially suited for growing conditions
in India. The resulting rice plants were not only tolerant of being
submerged in water but also produced high yields and retained other
beneficial crop qualities. Development of submergence-tolerant
varieties for commercial production in Laos, Bangladesh and India is
now well under way.

In addition to providing a more stable supply of rice in developing
countries, the researchers are hoping that the new gene will be
useful in suppressing weeds and reducing herbicide applications for
conventional and organic rice farmers in developed countries like the
United States. If water can be left on the rice for an additional
week, it is expected that weed populations will be reduced.

The research team is now trying to identify all the genes that are
regulated by Sub1A and to use this information to further improve
tolerance to flooding and other stresses.

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Funding for this study was provided by the U.S. Department of
Agriculture's Cooperative State Research, Education and Extension
Service - National Research Initiative; the U.S. Agency for
International Development; and the German Federal Ministry for
Economic Cooperation and Development.

In addition to Ronald and Mackill, this international research team
included Kenong Xu, Xia Xu and Patrick Canlas, all of UC Davis;
Takeshi Fukao and Julia Bailey-Serres, both of UC Riverside; and
Reycel Maghirang-Rodriguez, Sigrid Heuer and Abdelbagi Ismail, all of
the International Rice Research Institute in the Philippines.

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